Check valves are self-activating relief valves that operate entirely by reacting to the flow and pressure generated by the process streams in which they are installed. Also known as one-way directional or non return valves, these devices are designed to allow fluid, air or gases to flow in only one direction and eliminate backflow.
Large and miniature check valves are all non return valves that are employed in a number of industrial, commercial and domestic applications where they are responsible for the safe and consistent downstream flow of a number of materials including water, oxygen, fuel, acids, bases, sludge, slurry and even hazardous waste. Metal and plastic check valves may be used in variable applications.
It is important that the material construction be compatible with the chemical composition of the process stream. Relief valves, also known as PVC check valves are popular alongside brass check valves, since they both provide durable devices that can handle both chemical corrosion and constant wear. Selecting the proper valve configuration also helps to eliminate unnecessary wear and improves product longevity.
Ball check valves, lift check valves, swing check valves and wafer check valves all allow the continuous downstream flow of materials while blocking upstream contamination. When the differential pressure across the valve is high, spring check valves are used to prevent valve clapping and overflow due to heightened downstream flow.
They also protect against strong upstream surges such as water hammers. Double check valves, which use multiple valves in conjunction with one another, may also be employed to reduce wear and further protect against backflow that may otherwise result in pressure surges, cross-contamination and even flooding. Read More…
Check valves are becoming increasingly popular because of their simple design and ease of use. The mechanics are fairly simple. As aforementioned, check valve operations are determined completely by the process flow, meaning no additional actuating mechanisms are necessary. In general, the valve appears as a cylindrical device attached to the pump head on the inlet and outlet lines. Open in both ends, the working apparatus cross-cuts the housing, dividing it into an upstream and downstream portion. The seat extends from the walls of the cylinder but has an opening suitable to passing the process stream.
A ball, cone, disc or other oversized device rests against the seat on the downstream side of the check valve. Limited mobility keeps the plugging device from being swept downstream. When the stream moves in the pre-determined direction at the necessary pressure, the plug is dislodged from the seat, and fluid or gas is permitted to pass through the resulting gap. As the pressure drops, the stopper returns to the seat to prevent backflow.
Gravity or a spring loaded mechanism are commonly responsible for this return motion, though in some instances the increased pressure on the downstream side of the valve is enough to move the device back into place. The closing of the valve prevents downstream materials from mixing with upstream materials even under increased pressure. The specific stopper used is variable depending upon the type of check valve installed. As the name suggests, ball check valves use a ball. Lift check valves utilize a cone or disc attached to a rod like guide which ensures it will return to the proper place on the seat. Swing and wafer valves use one or more discs to seal the gap in the valve seat.
While the general design and function of the check valve is relatively simple, there are a number of important considerations that must be taken into account to ensure that materials flow through the valve and that backflow is adequately prevented. Check valves come in a range of sizes. The most important consideration regarding the size is the diameter of the seat and stopping device, which may be less than an inch or several feet across. Properly sized check valves are easily installed in pre-existing flow lines, using the dimensions of the pipe or tubing to determine the necessary size.
The valve size will also affect the pressure rating or cracking pressure of the valve. This is the minimum pressure at which the valve will operate, or open. If the valve is too large or bulky, the pressure created by the process stream will not be able to open it, even when the opposing pressure is exceeded. Undersized apparatus will allow a continuous flow and never close. Such malfunctions could result in damaged equipment, wasted materials and potentially hazardous cross-contamination.
The materials of construction also impact the effectiveness of check valves. If incompatible materials are used, they will wear, erode or contaminate the process stream. As a number of plastics, metals and synthetics may be used, compatibility is always attainable no matter the gas or fluid being transferred. Common materials include PVC and brass as well as ductile iron, copper, polyethylene, polypropylene, aluminum, steel, stainless steel and rubber. Further factors to consider include media temperature and the valve flow coefficient.
Each of the aforementioned considerations, as well as the specific type of check valve employed, depends largely upon the intended use for that valve. Virtually every plumbing or fluid transfer application imaginable, be it industrial, commercial or domestic, will employ check valves. In fact, check valves are an essential part of every day life. They are used in water heaters, indoor plumbing, faucets and dishwashers as well as more advanced equipment such as metering pumps, tankers, flowmeters, boilers, steam traps and industrial mixers and blenders.
Sewage, water treatment, medical, chemical processing, power generation, pharmaceutical, chromatography, agriculture, hydropower, petrochemical and food and beverage processing industries utilize check valves in the everyday operations of their facilities for effective backflow prevention. Because they prevent product malfunction and need not be supervised while functioning, check valves are not only desirable but often required by law to ensure the safety of water, gas and pressure applications.